Hmmm.
Looks like a resizing mark as per above. Incipient Case Separation would normally show as a very fine line slightly further up the case and happens when the case has been fired too often or with stout loads resulting in overstretching the brass. The line caused by resizing can be seen in the examples below and had nothing to do with case failure.
Always good reloading practice to check each case after resizing and before trimming or priming; easy to do with a bent and sharpened paperclip or a little tool - simply slide it up the inside of the case a few times whilst gently rotating the case - the pointed tip will snag on any internal cracks before they appear on the outside.
Simples…

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